Microwave and ultrahigh frequency electromagnetic energy have several clinical applications, including the induction of hyperthermia as an adjunct in the treatment of solid tumors, diagnosis of early pulmonary edema, and (the proposed) detection of breast cancer. In order to control the microwave heating of tissue, and predict the patterns of microwave absorption in tisses, one needs accurate values for the dielectric properties of various tssues. These data also can be interpreted on a molecular basis to yield important information about the physical properties of tissue water. In this study, we will measure the dielectric properties of various normal and tumor tissues and cell suspensions using a coaxial line technique and microwave network analyzer, over the frequency range 0.1 to 18 GHz, including the frequencies most useful for electromagnetic induction of hyperthermia. Tissues studied will include surgically excised tumors from living dogs, normal tissues from freshly killed dogs, lymphoma and ascites cell suspensions, and mouse spleen tissue. The net water content of each sample will also be measured. The dielectric data from these soft tissues will be interpreted using the Maxwell moisture theory, and compared to the known properties of pure water in this frequency range, to yield (a) the apparent amount of "bound" water, and (b) the rotational mobility of the major fraction of water in normal and tumor tissues. This analysis will permit one to predict the dielectric porperties of soft tissue from a single measurement of the net water content of the tissue, and compare the physical properties of water in normal and tumor tissues.